In fiber-optic communications, an optical add-drop multiplexer (OADM) is an optical node used in wavelength-division multiplexing (WDM) systems for multiplexing and routing different channels of light into or out of a single mode fiber. Typically, an OADM has the capability to add one or more new wavelength channels to an existing multi-wavelength WDM system; and to drop, or remove, one or more wavelength channels from the multi-wavelength WDM system by passing the dropped signals to another network path. A reconfigurable optical add-drop multiplexer (ROADM) is a type of OADM that enables an operation to remotely switch traffic in a WDM system at the wavelength layer without manual intervention. In “multi-degree” ROADM design, the number of line output ports to which a channel may be routed (or alternately the number of line input ports from which a channel may be routed) is considered the number of degrees. Signal addition and removal is not considered a degree.
Contentionless routing refers to the ability to add/drop multiple instances of the same channel wavelengths at the same ROADM node. Contentionless ROADMs are of interest as a means of ensuring fully flexible reconfigurable add/drop at an optical node, providing important operational benefits to a carrier. The number of these instances that can be supported at the same wavelength on either add ports or drop ports is referred to as the “contentionless degree” of the ROADM. When the contentionless degree of a ROADM is equal to or greater than the degree of the ROADM it is considered to be fully contentionless. However, implementation of such a fully contentionless ROADM may be cost prohibitive, especially for multi-degree nodes of high degree, due to factors including, for example, the amount and complexity of additional switching required, increased port count requirements for existing switch elements (e.g., wavelength selective switches), and for some architectures a need to provision a larger number of available transponders in preparation to quickly turn up add/drop traffic.
However, in the midst of relentless growth in bandwidth demand, specific traffic patterns for optical networks have proven to be difficult for carriers to predict. A node that begins as a few-degree node may unexpectedly need to quickly grow to become a many-degree node, and the amount of add/drop required at a particular node may also change unpredictably over time.